1. Field of the Invention
This invention relates to the field of electrochemical cells, and, in particular, to the field of carbon-containing anodes for such electrochemical cells.
2. Description of Related Art
Many different carbon materials have been investigated for use in anodes for lithium/carbon electrochemical cells (i.e., electrochemical cells having anodes comprising at least partially crystalline carbon and "intercalated lithium"). Intercalated lithium is lithium that is bonded to the carbon in the carbon crystals by van der Waals forces. Key requirements for the carbon in such electrochemical cells are that the carbon has a high capacity both for reversible intercalation of the lithium during charging the cell and for deintercalation of the lithium during discharging the cell and that this high capacity be maintained with repetitive cycling.
Coke and graphite are two of the most commonly cited carbon materials for use in lithium/carbon electrochemical cells. Graphite is preferred for capacity since it can intercalate one lithium atom for every six carbon atoms (denoted as "LiC.sub.6 "). This corresponds to a deliverable electrochemical capacity of 372 mAh/gm (milliampere hours per gram) of carbon. However, graphite is known to show a high reactivity with many of the liquid electrolytes used in lithium/carbon electrochemical cells, resulting in substantial irreversibility and a continual loss in cell capacity on cycling. This effect can lead to a complete loss in capacity for an electrochemical cell.
Coke can generally intercalate one lithium atom for every 12 carbon atoms (denoted as "LiC.sub.12 ") because coke has a less ordered crystalline structure than graphite. The degree of intercalation can also decrease with repetitive cycling for cokes but to a much lower extent than graphite. Therefore, the maximum sustainable capacity that has been achieved with coke is about 186 mAh/gm. By way of illustration, Guoyomard and Tarascon, J. Electrochem, Soc. 139, p. 938 present several references for the use of "Conoco electrode coke" which gives only "50% LiC.sub.6." where "% LiC.sub.6 " denotes the percent of the lithium atoms in the carbon that are intercalated with six carbon atoms (i.e., "50% LiC.sub.6 " denotes LiC.sub.12 or 1/2 LiC.sub.6). M. Jean et al., J. Electrochem, Soc. 142, 2122 (1995) report 52% LiC.sub.6 for coke. Japanese disclosure HEI-204361 shows low capacity (less than 100 mAh/g) for Conoco coke and pitch cokes.
It would be desirable to utilize a variety of coke as an anode in lithium/carbon electrochemical cells that would exhibit both high stability with respect to liquid electrolytes and an electrochemical capacity closer to that of graphite.
U.S. Pat. Nos. 5,162,170 and 5,176,969 disclose lithium secondary batteries stated to have a large electrode capacity, excellent charging and discharging cycle characteristics and flexibility wherein the negative electrode has an active substance carried on a carbonaceous material and a binder or a conductive polymer. In the carbonaceous material, other atoms such as nitrogen, oxygen and halogen may be also present at a ratio preferably of 7 mole percent or less, more preferably 4 mole percent or less, particularly preferably 2 mole percent or less. In the case of nitrogen, the foregoing mole percentages corresponds to weight percents of 2.8 weight percent, 1.6 weight percent and 0.8 percent. These patents disclose no specific exemplification of the use of any carbonaceous material containing nitrogen in a secondary battery.
A subsequent publication [Behavior of Nitrogen-Substituted Carbon (N.sub.z C.sub.1-z) in Li/Li(N.sub.z C.sub.1-z).sub.6 Cells by W. J. Weydanz, B. M. Way, T. van Buuren, and J. R. Dahn, J. Electrochem. Soc., Vol. 141, No. 4, April 1994, 900-907] discloses that nitrogen-containing carbons have been made from different precursors at temperatures between 850.degree. and 1050.degree. C. This publication discloses carbons containing from 0 to 8.53 percent nitrogen by weight and concludes that such nitrogen-containing carbons are not considered useful as anodes for lithium-ion cells.